Wireless communication networks of various types use forms of Automatic Repeat reQuest (ARQ) response signaling. With ARQ, transmissions from a given transmitter are acknowledged or not acknowledged, depending on whether they are successfully received. Non-acknowledgments prompt the transmitter to retransmit using, for example, the same channel resources that were allocated for its original transmission.
According to the LTE standards, as promulgated by the Third Generation Partnership Project (3GPP), LTE networks use Hybrid ARQ (H-ARQ). As an example, given mobile terminals or other types of User Equipment (UE) transmit to an eNodeB in one or more LTE subframes, according to uplink assignment grants made by the eNodeB. An assignment grant allocates particular OFDM channel resources to particular users. Thus, in ongoing operation, the eNodeB receives some number of user signals in each of a series of repeating LTE subframes, and sends ARQ responses to each transmitting user, in dependence on whether that user's signal was successfully received (decoded) by the eNodeB.
In more detail, in 3GPP Release 8, for LTE, an eNodeB sends an H-ARQ response signal on the Physical H-ARQ Indicator Channel (PHICH), wherein an acknowledgment response—sometimes referred to as an “ack”—indicates that the user signal transmitted on the uplink to the eNodeB by a given User Equipment (UE) was successfully decoded. Conversely, a non-acknowledgment response—sometimes referred to as a “nack”—indicates that the user signal was not successfully decoded.
In Frequency-Division Duplexing (FDD) mode, the eNodeB receives some number of user signals in a given LTE subframe, and sends ARQ response signals corresponding to those signals four subframes later, as a PHICH group transmitted on the PHICH. The determination of the PHICH group, as well as the different spreading sequences used to differentiate the different ARQ responses by targeted UEs, is determined based on the locations of the corresponding uplink assignments used for the transmissions being acknowledged. The PHICH is mapped on OFDM symbol “0” for normal durations, or 0, 1, and 2 for extended durations.
To efficiently utilize the available resources, the ARQ responses (ack or nack signals) for up to eight UEs can be multiplexed into a single PHICH group, and there are several such PHICH groups available. The number of PHICH groups depends on the system bandwidth and a semi-static parameter called Ng that dynamically accounts for changes in the number of users. There are at least two PHICH groups (1.4 MHz and Ng=⅙) and at most twenty-five (20 MHz and Ng=2) present in the control region of an LTE subframe.
The LTE standards define the target probabilities for a given UE misinterpreting an ack signal as a nack signal, and vice versa. The target probabilities are defined in terms of Bit Error Rates (BERs) or Block Error Rates (BLERs), for example.